Our long-term objective is to elucidate basic cellular and molecular mechanisms of synapse specification during neural development. In this application, we propose to test using reverse genetics whether nematode cadherins and/or leucine-rich repeat proteins (LrrCAMs) can satisfy the ?chemoaffinity hypothesis? for synaptic target recognition. The adult C. elegans hermaphrodite has a total of 302 neurons representing 118 distinct types; they form some 5000 synapses and 600 gap junctions (White et al., 1986). Comparisons of reconstructed individuals reveal a core circuit of stereotyped connections and provide unequivocal evidence for chemoaffinity restrictions in synapse formation in addition to the various pathfinding mechanisms that bring neurons into contact (Durbin, 1987). The comparative simplicity of the C. elegans nervous system facilitates genetic analysis of synapse patterning. The small size and simplicity of the neuropil allow in vivo imaging of identified synapses arid practical EM reconstruction. Despite its size, the nervous system has a remarkable diversity of neuron types and behaviors (Bargmann, 1993). Several recent advances make functional genomics, or reverse genetics, an attractive complement to classical genetics of synapse patterning. The genome of C. elegans is comparatively small and several gene families implicated in synapse formation, e.g., cadherins and LrrCAMs, are modest in size (Hutter et al., 2000). Finally, the discovery of potent and specific genetic interference using dSRNA has provided an easy and general ?knock-out? approach to gene function (Fire et al., 1 998; Tavernarakis et al., 2000). Here we propose six specific aims. Aims 1 & 2 survey the developmental expression and possible neural functions of all forty members of the C. elegans cadherin and LrrCAM superfamilies. Aims 3, 4 & 5 study the products and functions of selected genes in greater detail. Aim 6 asks directly whether cadherins or LrrCAMs have instructive roles in synapse patterning.